The invention generally relates to follower circuits, and more particularly relates to class AB and super follower circuits.
Follower circuits are commonly used as drivers to buffer signals and provide a low output impedance to drive resistive (R) and/or capacitive (C) loads. Follower circuits typically exhibit high linearity and low distortion characteristics. However, in applications where the C or RC load is large, follower circuits have the disadvantage of consuming much power and requiring a larger area. Furthermore, to increase bandwidth, a follower circuit requires large bias current/transistor size to increase transconductance (gm) and reduce output impedance. The transconductance problem may be particularly troublesome in certain types of applications, such as, for example, in deep sub-micron complementary metal oxide semiconductor (CMOS) processes. Increasing transistor size further contributes to the problem of increased C or RC loads, which leads to even more power consumption and larger area.
Follower circuits also have the disadvantage of larger slew rates or settling times. In electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit. Limitations in slew rate capability can give rise to non linear effects. To reduce the large signal settling time, follower circuits may require larger bias current, which in turn increases power consumption and silicon area.
To address these and other problems, certain types of follower circuits have been developed. For example, class AB followers address the problem of increased slew rates by implementing a push-pull driver effect. The push-pull arrangement is provided with a complementary pair of transistors, in which each transistor amplifies opposite halves of the input signal and, thereby, charges and discharges the C or RC load. While this arrangement addresses the slew rate problem, class AB followers have significant disadvantages. Class AB followers require large headroom (e.g., higher power supply) for biasing and do not adequately address the need for low output impedance.
Another type of follower circuit is a super follower circuit. Super follower circuits address bandwidth issues by reducing output impedance with a gain path that generates high current drive (transconductance). While super follower circuits can meet bandwidth requirements (e.g., by reducing follower impedance), they do not adequately address the slew rate problem. Super follower circuits generate high current drive only on the side of the gain path, and still have to overcome the current from the other side. In other words, super follower circuits may either address the rise time or the fall time—not both.
Despite the many advantages and the commercial success of class AB followers and super followers, there remains a need in the art for follower circuits that have the combined advantages of low output impedance, high bandwidth, and push-pull capability to reduce transient settling time.